Conventional broadcast NTSC composite color subcarrier television signals nominally occupy assigned broadcast channels which are 6 MHz wide. The visual (AM) carrier frequency is nominally 1.25 MHz above the lower boundary of the channel. The aural carrier frequency (FM) is centered 4.5 MHz above the visual carrier frequency; and the chrominance subcarrier is 3.579545 MHz above the visual carrier frequency.
As is well known, since the color subcarrier frequency an odd multiple of one half of the line scan rate, the sideband signals carrying the chrominance information exist as energy groups which are interleaved within the high frequency energy groups of luminance. When the chrominance and luminance energy groups overlap spectrally, the resultant picture display may be characterized by cross-color artifacts and cross-luminance artifacts. Such artifacts may be noticeably reduced by comb filter encode and decode processing, such as is taught in the present inventor's U.S. patent application No. 07/017,166 filed on Feb. 20, 1987, now U.S. Patent No. 4,731,660.
Recently, with the advent of improvements in television picture display systems, workers in the art have proposed a wide variety of improved resolution television systems, one of which is known as "high definition television or `HDTV`" which proposes new transmission standards, typically 1050 scan lines, which are not immediately compatible with existing television receiving devices. Scan line doubling techniques, such as described in U.S. Pat. No. 4,719,644, are also known in the prior art as a way to increase virtual resolution.
One other, pertinent proposal is that set forth in an article co-authored by Rzeszewski, Pazarci and LoCicero entitled "Compatible High-Definition Television Broadcast Systems", IEEE Trans. on Broadcasting, Dec. 1987 issue. That article sets forth systems using two standard, adjacent 6 MHz channels and which were capable of increased horizontal and vertical resolution, a wider aspect ratio and reduced cross-talk between the various components of the composite signal. The luminance signal is given the benefit of over 7 MHz of bandwidth, which results in a much sharper picture image display. Thus, it is presently known to employ a 12 MHz spectrum for carrying a bandwidth increased, NTSC compatible, quadrature modulated color subcarrier signal. Resolutions approaching those obtainable with 35 millimeter format film are realistically obtainable with some of these bandwidth extending techniques. In such two-channel systems it is desirable that the channels be adjacent channels (a situation realistically available in the cable television industry, and also in the UHF broadcast spectrum). Also, it is highly desirable that the two-channel signal be capable of being received on conventional television receivers with little or no apparent degradation in picture quality. Further, it is highly desirable that luminance and chrominance components be separated with e.g. comb filter processing so that cross-color and cross-luminance artifacts are thereby eliminated and thereupon provided as separate outputs to the more recent "S-VHS" television receivers having the capability of receiving and processing luminance and chrominance separately. It is also desirable that the two-channel system provide composite video and modulated RF outputs.
The present inventor has discovered increased resolution provided by two-channel television systems solves but one aspect of a multi-faceted problem. Most of the facets of this problem do not relate to resolution. Rather, most relate to unwanted noise and interference artifacts, such as short term ringing and echo due to vistigial sideband filter group delay errors, and ghosts from multipath transmission phenomena.
In cable television systems the apparent defects of the NTSC format, such as cross-color, cross-luminance artifacts and lack of resolution are of less concern than the more pressing problems of adjacent channel interference, noise and short term ghosts and ringing (300-500 nanosecond duration) due to deficient line terminations associated with distribution amplifiers in local distribution loops. Existing cable television systems lend themselves directly to the use of two adjacent 6 MHz channels for pay television services offering the highest in technical quality, such as movie channels, where it is desirable to reproduce pictures with a quality comparable to the picture quality experienced in theaters.
Thus, a hitherto unsolved need has existed for a wide RF bandwidth (12 MHz) television spectrum which is compatible with the NTSC format spectrum and which also delivers an image which is free of noise, interference, and short term ringing.